The present invention relates to time-temperature indicators and, more particularly, to a time-temperature indicators of the type incorporating thermochromic material and to a method for determining the internal temperature of an object by use of the time-temperature indicator.
The invention is particularly useful in cooking, canning or sterilization. For these purposes it is advantageous and often essential that the temperature at the internal area or volume of an object be determined. As for example, in boiling an egg it is known that egg white and egg yoke coagulate at about 60.degree. to 70.degree. C. Recommended practice is to immerse the egg in cool water and to apply heat. Heating is continued for a preselected length of time after boiling begins. The time is selected on the basis of experiment or experience. However, with the usual method there is really no predictability as to the hardness or softness of the egg. The result depends upon many factors which have not been accounted for. Examples of such factors include variations in the initial temperature of the egg or other object, the burner setting during heating, the heat content of the fuel or the voltage of an electric heating element, or changes in altitude which cause variations in the boiling point of liquids. All of these variables affect the time required for the internal area of the object to reach the desired temperature. The internal state of the object being subjected to temperature change is at best an approximation.
For certain uses it is desirable that the temperature indicator be reversible so that it can be reused. It is also desirable often times to determine that the temperature has dropped throughout the entire volume of the object. For example, it would be undesirable to store some objects in a refrigerator until the temperature dropped to a reasonable level. In other applications, it is advantageous to retain a permanent indication that a desired temperature has been attained throughout the entire volume. This would be true for sterilization and canning, for example. In such cases an irreversible indicator may be suitable to provide a permanent recording of the fact that the heating process has been completed.
Temperature indicating devices using thermochromic material are known in the prior art. These utilize thermochromic material in which the material goes through a spectrum of color changes at different temperatures. They sometimes incorporate thermochromic materials deposited in the shape of numbers which change colors and become visible at the temperature indicated by the particular number. Others are sheet material which give digital readouts or color changes at pre-determined temperatures depending on the precise composition of the material. However, none of the prior art thermochromic temperature indicators are capable of determining internal temperatures after a given exposure time to a change in ambient temperature at the surface. Certain devices of this type are known as, for example, meat thermometers which have pointed probes and can be thrust toward the center of the object being heated. Candy thermometers include a probe which can be inserted into the liquid being heated. However, certain objects cannot have their internal temperature measured in this way without destroying the object. An egg is one example. A sealed container which is undergoing heating for sterilization of the material within it is another example. In canning it is the practice to maintain the lid loosely on the container so that the container can be submerged in the boiling liquid. Use of a thermometer in a container would not be feasible.